Field of the Invention
The present invention is concerned with a method and an apparatus for resting state functional magnetic resonance imaging (rsfMRI).
Description of the Prior Art
Magnetic resonance imaging is an imaging modality that makes use of the fact that different types of nuclei are resonant at respectively different frequencies in a magnetic field of a given field strength. Each type of nuclei exhibits a property known as the gyromagnetic ratio, which causes that nucleus to resonate at a specific frequency in the presence of a magnetic field of a specific field strength. The nuclei are initially aligned by the strong magnetic field, and, by the application of radio-frequency (RF) energy thereto, are deflected by an angle (called the “flip angle”) from the aligned state. As the deflected nuclei return to the aligned state, they emit RF signals (magnetic resonance signals) which are detected and processed in order to generate a magnetic resonance image.
A special category of magnetic resonance imaging is functional magnetic resonance imaging (fMRI). As explained in U.S. Pat. No. 7,349,728 (the teachings of which are incorporated herein by reference), functional magnetic resonance tomography makes use of the fact that the oxygen content of blood influences its magnetic properties. The magnetic resonance characteristics, and thus the magnetic resonance signal generated by blood, change with the content of oxygenated or de-oxygenated hemoglobin. Therefore, blood behaves in functional magnetic resonance tomography in the manner of a contrast medium. With a high proportion of de-oxygenated hemoglobin, as a result of its paramagnetic characteristics in the environment of the blood vessels, a local magnetic field gradient is induced which, with a suitable choice of a magnetic resonance tomography sequence, a localized signal reduction will occur. If the proportion of oxygenated hemoglobin in the blood increases, an effect known as the susceptibility effect decreases, which leads to an increase in the measured signal. This relationship is referred to as the BOLD (blood oxygen level dependent) contrast, or BOLD effect. With increasing field strength, this effect is increased. Magnetic resonance devices (scanners) that generate a basic magnetic field strength of 1.5 Tesla and higher are used for fMRI.
The magnetic resonance sequence that is typically used in fMRI is an echo planar imaging sequence, so that fMRT is a type of echo planar imaging.
The local changes of the oxygen content in the blood can be caused, for example, by intentionally subjecting the patient to one or more sensory inputs (such as light, sound or touch) at known times, and the fMRI image will enable a physician to identify the location in the brain at which increased brain activity occurs as a result of the stimulus.
Resting state fMRI (rsfMRI) is a form of fMRI wherein BOLD signals are acquired from a subject with no external stimuli being applied. The BOLD signals are then statistically analyzed to determine degrees of connectivity between various regions of the brain. In general, the greater that a low frequency (such as below approximately 0.1 Hz) modulation of the BOLD signals between two brain regions is correlated over time, the higher the respective connectivity between those two regions.
This technique is robust with individual patients, and allows the same scan to be used to survey multiple brain systems, in contrast to conventional task-based BOLD examinations. Investigations have shown that rsfMRI can be used to identify brain systems that are associated with motor function, and associated with cognition, including language and memory. Major clinical applications include early stage diagnosis of Alzheimer's disease, grading depression severity, pre-surgical planning, and transcranial magnetic stimulation (TMS) targeting. In clinical practice, rsfMRI sequences are commonly used in conjunction with structural or anatomical (morphological) sequences, such as a 3D MPRAGE (Magnetization Prepared Rapid Gradient Echo) sequence. Conventionally, data are acquired from the examination subject using an rsfMRI sequence, and then a separate examination takes place wherein data are acquired from the subject using a morphological sequence, such as MPRAGE. For example, in early stage diagnosis of Alzheimer's disease, rsfMRI data are combined, after such separated acquisitions, with morphological MRI data, from which brain atrophy can be identified from the structural sequence. In pre-surgical planning for temporal lobe epilepsy and tumor reception, and TMS targeting for depression, the functional measurements acquired from the rsfMRI data are directly referenced to the anatomical structural data acquired from the morphological sequence from the same patient.
In view of the need to acquire data both in a structural sequence and in an rsfMRI sequence, the overall measurement (data acquisition) time can be prolonged so as to have a duration of 15 to 30 minutes. This can present a significant problem in the case of elderly patients, demented patients, or severely depressed patients. It is thus of major interest to significantly reduce the total measurement time in these types of examinations.